1. Field of the Invention
The invention relates in general to the fabricating method of semiconductor integrated circuits (ICs), and more particularly to a method of forming a high-voltage device.
2. Description of the Related Art
A high-voltage device is one of the most important devices utilized in highly integrated circuits. In an integrated circuit, high voltage devices such as electrically programmable read-only memories (EPROM) or flash memories are often included. While programming a flash memory, the source is typically grounded or zero biased, whereas a voltage of about 12V is applied to the gate and a voltage of about 7V is applied to the drain. Under this condition, a hot electron injection is very likely to occur in the channel region between the drain and the source. The mechanism of the hot electron injection is the Fowler-Nordheim (F-N) tunneling effect. For a reading operation, a voltage of about 5V is applied to the gate, whereas the voltage applied to the drain is required to be higher than that to the source. Furthermore, while erasing, the drain is in a floating status, whereas the gate is grounded and a voltage of about 12V is applied to the source. In general, operating a device under high voltage can increase the speed of a read or a write operation. Thus, a high-voltage device, which can be operated under high voltage, are required in integrated circuit.
Due to the increasing number of semiconductor devices incorporated in integrated circuits, the size of transistors needs to be decreased. Accordingly, as the channel length of the transistor is decreased, the operating speed is increased. However, there is an increased likelihood of a problem, referred to as a "short channel effect", caused by the reduced channel length. If the voltage level is fixed, according to the equation of electrical field=electrical voltage/channel length, then, as the channel length is shortened, the strength of the electrical field is increased. Thus, as the strength of the electrical field increases, energy of electrons increases and electrical breakdown is likely to occur.
In a conventional high-voltage device, occurrence of potential crowding on the edge of the drain region decreases breakdown voltage. In order to increase the breakdown voltage and make the device operable under high voltage, the doping concentration of the drift region must be decreased. Unfortunately, as the doping concentration of the drift region decreases, the current driving performance decreases.
In the conventional high-voltage device, the formation of an isolation layer is used for the purpose of increasing the channel length. In addition, a lightly doped ion implantation is performed on the junction between a depletion region and a source/drain region in order to decrease the hot electron effect. In this way, the breakdown voltage of the source/drain region increases. The high-voltage device is able to work normally under a high electrical voltage.
FIG. 1 is a schematic, cross-sectional view showing a portion of a conventional high-voltage device.
In FIG. 1, a field oxide layer 102 is formed in the P-type silicon substrate 100. A gate oxide layer 104 is formed on the substrate 100 and beside the field oxide layer 102.
The field oxide layer 102 is used to increase the channel length between an N.sup.+ -type source region 106 and an N.sup.+ -type drain region 108. An N.sup.+ -type lightly doped region 110 is under the N.sup.+ -type drain region 108 and under the field oxide layer 102. A P.sup.- -type lightly doped region 112 is under the source region 106. A portion of a gate 114 is on the gate oxide layer 104 and the other portion of the gate 114 is on the field oxide layer 102. The N.sup.- -type lightly doped regions 110 and the P.sup.- -type lightly doped region 112 are used as drift regions for carriers while the device is operated. As a junction depth of the N.sup.- -type lightly doped regions 110 and the P.sup.- -type lightly doped region 112 increases, the effective channel length decreases. There is also a P-N junction (not shown) formed between the N.sup.- -type lightly doped region 110 and the P-type substrate 100. The P-N junction region are called depletion region. The electrical distribution lines, which are nearby the channel, of the N.sup.+ -type lightly doped drain region 108 have higher curvature while the device is operated under high voltage. The breakdown voltage thus is decreased. Consequently, the device cannot work normally under high voltage.